Victoria, Patrick, great that you were aware of that! cause, last time i
talked with someone (manfred, maybe?), I myself was not aware of it
anymore. realising that is why I sent this note in the beginning...

regarding Patrick's question - at least the intensity component is getting
smoother with more scattering, i.e. the interpolation does not get more
serious with stronger scattering (i don't have a good feeling for higher
stokes components. there it might depend, e.g. on the surface properties.).

My main concern comes from the interplay of RT4 with ARTS: Using the
auto-increase-streams feature, first the radiation field is calculated with
high(er) angular resolution (as high as the phase-function norm
reproduction (aka energy conservation) requires), then this
high-angular-resolution-field is interpolated onto the low-resolution
original angular grid in the RT4 interface and last, in
(i)yCalc/iyEmissionStandard, the radiation value at a given angle is
extracted by interpolation.

It's this 2-step interpolation with the first one degrading resolution that
worries me (it feels safer to have only one interpolation step. or,
alternatively, to at least go from low to high resolution in the first
step, not the other way round. either requires some additional internal
logistics, though - which is why it hasn't been done like that yet.)

Moreover, the iyEmissionStandard interpolation is hardcoded to a linear
interpolation (in angle space) so far - which should maybe be replaced in
any way, since this is particularly bad; both higher order polynomials and
interpolation in cos(angle) space seem (!) to give better results.
Interpolation in the RT4 interface can be controlled by the user, selecting
the interpolation order and the interpolation space (angle or cos(angle));
the default so far, though, is also linear in angle space (for consistency
with iyEmissionStandard and because it's not very clear, which setup is
best. or consistently good. would need a bit more testing... if anyone has

Victoria, I get back to you separately regarding your specific questions.

best wishes,

On Wed, Jan 24, 2018 at 8:00 PM, Victoria Sol Galligani <
victoriasgallig...@gmail.com> wrote:

> Hello everyone! Hi Jana!
> I was aware of what you are saying about the number of streams. However
> .... in the context of the ARTS scattering methods + RTTOV-scatt
> inter-comparison I am working on, I have been testing the sensitivity of
> the output TBs to the number of streams chosen. In this regard, I think it
> would be interesting to run RT4 even if the interpolation induces large
> errors and the scattering matrix is not resolved well. If I run RT4 without
> this warning (actually error because arts doesn't run with its current
> checks) I could easily answer your answer Patrick (if the angle
> interpolation error increases with strong scattering), because I'm running
> with different profiles, some with much more scattering than others. At the
> moment its clear to me that more streams are needed for more scattering
> cases, so I guess that already answers your question Patrick ...
> Do you have any advice regarding turning this warning/error off on my arts
> distribution?
> Looking forward to sharing my preliminary results on this subject,
> Hugs,
> Victoria
> On Mon, Jan 22, 2018 at 4:56 PM, Patrick Eriksson <
> patrick.eriks...@chalmers.se> wrote:
>> Hi Jana,
>> I was aware of that the auto-increase does not change the number of
>> output angles, but still thanks for the warning.
>> That is, for me it was understood that you should check that the start
>> number of streams is high enough, to make sure that angle interpolation
>> does not induce too large errors. But not clear to me is if the angle
>> interpolation error increases with strong scattering. This is something
>> that I have never tested.
>> Your comments seem to indicate that it is the case (i.e. increasing
>> errors), but has you tested it? Without that it is a bit hard to decide
>> what to do.
>> Anyhow, my recommendation is that you focus on general cleaning and
>> documentation. Leave any possible extension of RT4 on this point to us
>> others, if we find it necessary.
>> Bye,
>> Patrick
>> On 2018-01-22 17:46, Jana Mendrok wrote:
>>> Hi,
>>> With some of you we have discussed (even suggested) the use of the
>>> auto-increase number of streams feature of the RT4 interface.
>>> The background to this feature is that RT4 needs the scattering matrix
>>> to be properly resolved in order to conserve the scattered energy
>>> satisfactorily. Using the feature, the number of streams is internally(!)
>>> increased until the scattering matrix resolution is deemed sufficient.
>>> The crucial issue, i just got reminded of when i went through the code,
>>> is that this increase is only done internally. the output will remain on
>>> the original number of streams set!
>>> That means, *one should not start with a very low number of streams* and
>>> should not let the system completely self-adapt (as, strictly speaking,
>>> that's not what it is doing - the output dimensions won't adapt and will
>>> always remain the original/starting number of streams) (i'm going to add
>>> that info to the online doc).
>>> It should be kept in mind, that - unlike Disort - neither RT4 nor ARTS
>>> itself have good interpolation options for "off-stream" angles, i.e. the
>>> number of streams RT4 is set up with does not only determine the RT4
>>> solution accuracy (this is improved/adapted with the auto-increase
>>> feature), but also the number of output directions (not affected by
>>> auto-increase), hence the accuracy with which the field is known to and
>>> further applied within other WSM of ARTS.
>>> Best wishes,
>>> Jana
>>> Ps.  Something more for developers...
>>> Thinking about that, this seems quite inconvenient. So, the question
>>> what to do about it, how to change that. Two possible solutions pop into my
>>> head:
>>> (1) instead of interpolating the high-stream-solution to the low number
>>> of streams, we could interpolate everything to the highest number of
>>> streams and output the "high-resolution" field.
>>> (2) re-define (doit_)i_field from Tensor7 into a ArrayOfTensor6 with one
>>> Tensor6 entry per freq. this would allow to have different angular
>>> dimensions per frequency (we'd need to also store the angles per each
>>> frequency). however, that would affect the output of other solvers, too,
>>> and the way (doit_)i_field is applied in (i)yCalc.
>>> so, option (1) seems less of a hassle.
>>> neither option will solve all issues (like, even if the radiation field
>>> is quite smooth, linear interpolation from low-resolution fields won't be
>>> very good and higher-order interpolation intrinsically requires, well,
>>> higher numbers of streams), but at least some (like better conserving the
>>> shape of the radiation field, where derived from higher number of streams).
>>> any opinions? do you consider this an issue at all, or is a cautionary
>>> note in the documentation enough? if an issue, any better ideas on
>>> solutions or opinions on "my" two options?
>>> and, anyone other than me willing to implement possible changes?
>>> --
>>> =====================================================================
>>> Jana Mendrok, Ph.D.
>>> Email: jana.mend...@gmail.com <mailto:jana.mend...@gmail.com>
>>> =====================================================================

Jana Mendrok, Ph.D. (Geoscience)

Email: jana.mend...@gmail.com
Phone : +46 (0)708 860 729
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